(1) Field of the Invention
The present invention relates to a method of stopping an engine in overspeed, to such an overspeed safety system, and to an aircraft including such an overspeed safety system.
The invention relates to the general technical field of systems that provide operating safety for aircraft engines, and in particular operating safety for a rotorcraft turboshaft engine.
The present invention relates more particularly to protecting a power plant in the event of an engine suffering overspeed.
(2) Description of Related Art
A rotorcraft includes at least one engine for rotating at least one rotor. Such a rotor provides said rotorcraft with at least part of its lift, or even its propulsion. By way of example, a twin-engine rotorcraft generally includes first and second engines acting together to drive a main rotor providing lift and/or also propulsion via a power drive train.
The first and second engines are generally controlled by respective first and second control units. Such control units are known by the acronym FADEC for “full authority digital engine control”.
Each control unit may comprise a computer and peripheral monitoring and control units constituting interfaces between the cockpit and the associated engine of the aircraft.
The engines may be turboshaft engines.
A turboshaft engine conventionally comprises a gas generator. The gas generator comprises in succession at least one compressor, a combustion chamber, and at least one expansion turbine, the compressor being mechanically linked to the expansion turbine by a drive shaft.
In addition, the turboshaft engine comprises a power assembly located downstream from the gas generator. That power assembly is sometimes provided with at least one power turbine set in rotation by the outgoing gas leaving the gas generator. The power turbine rotates a drive shaft engaging with the power transmission train that is connected to a rotor.
The power turbine is said to be “free” when said turbine is not connected by a shaft to the gas generator.
In particular, in the event of the power transmission train breaking or in the event of a freewheel in the power train slipping, the speed of rotation of the free turbine of a turboshaft engine can increase considerably. As from a threshold, the person skilled in the art then considers that such a turboshaft engine is suffering overspeed, where such overspeed can lead to the speed of rotation of the engine running away and to the engine bursting.
Consequently, aircraft are generally provided with safety systems so as to try to limit the impact of such overspeed on the aircraft and its occupants.
A mechanical system for providing protection against blade-shedding may sometimes be used for an engine with a free turbine.
This mechanical system comprises a ring of shielding arranged around the free turbine. In addition, each blade of the free turbine is fastened to a hub by a fuse element.
Beyond a threshold speed of rotation, the fuse elements break. Each blade is then separated from the hub. However, these blades remain contained inside the engine as a result of the shield.
The ejection of the blades from the free turbine causes the speed of rotation of said free turbine to slow down, and thus makes it possible to stop overspeeding of the free turbine.
Nevertheless, the engine is partially destroyed as a result of the blades being ejected. Furthermore, stopping the rotation of a free turbine does not necessarily cause the gas generator of the engine to stop.
Consequently, an electronic shutdown system can be implemented to stop turboshaft engines in order to prevent overspeeding.
The manufacturer of a turboshaft engine then establishes for example a threshold for the speed of rotation of the free turbine of said turboshaft engine. When that threshold is reached, the control unit stops the engine. In that configuration, the engine is not damaged by the safety system.
Document FR 2 962 165 suggests comparing only the engine torque transmitted by a free turbine with a torque threshold.
Nevertheless, the use of a single speed or torque monitoring threshold can cause untimely shutting down of the engine, e.g. when the rotorcraft performs a severe maneuver. Under such circumstances, such a monitoring system is sometimes not arranged on a single-engine aircraft.
On a twin-engine rotorcraft, engine shutdown can be inhibited in the event of the other engine already being shut down because of overspeeding, in order to avoid untimely shutting down of both engines. After a first engine has shut down, the stopping in flight of the second engine is then no longer authorized.
Under such circumstances, the second engine cannot, however, be stopped in the event of overspeeding. Such a situation is improbable, but not impossible. Consequently, if the second engine is subsequently in an overspeed condition, the second engine cannot be shut down automatically because its protection is inhibited. The second engine thus risks being in an overspeed situation.
In a variant, a pilot may then manually reset an electronic protection system against the overspeed of the second engine in order to avoid such a situation.
In this context, the turboshaft engine of a single-engine rotorcraft is sometimes fitted with a mechanical protection system of the “blade shedding” type.
However, a single-engine rotorcraft is not generally fitted with an electronic system for avoiding untimely shutting down of the only engine of the aircraft.
Twin-engine rotorcraft may however include a mechanical protection system of the “blade shedding” type for each engine, and an electronic shutdown system.
In addition, the technological background comprises a protection device that shuts down an engine in overspeed providing that a comparison is performed between a mechanical power required by the rotorcraft and a predefined power threshold. The mechanical power required by the rotorcraft is calculated depending at least on the current or anticipated value for the opposing torque of a main rotor of the rotorcraft.
Document FR 2 967 213 describes a method of controlling an overspeed safety system for an aircraft having at least two engines. That method consists in setting the overspeed safety system for the engines, in monitoring the speeds of rotation of the engines, in detecting overspeed on one of the engines, in shutting down the engine in question in the event of detecting overspeed, and in inhibiting the operation of the overspeed safety system for the other engine(s) still in operation. The overspeed safety systems of the engines still in operation can be reset as a function of one or more safety parameters.
Document FR 2 980 174 and document EP 2 570 617 describe a method of controlling an overspeed safety system for an aircraft having at least two engines. In that method, a first engine is shut down when a monitoring parameter of said first engine exceeds a first threshold, and a second engine distinct from said first engine is shut down when the monitoring parameter of said second engine exceeds a second threshold, the second threshold being above said first threshold.
In particular, the monitoring parameter is the speed of rotation of the free turbine of a turboshaft engine.
Documents U.S. Pat. No. 4,625,510, WO 93/04418 and U.S. Pat. No. 5,775,089 are far removed from the invention.